Coherent CoMP (Coordinated Multi-Point) transmission on the downlink significantly improves spectral efficiency and cell-edge bitrate of cellular systems. Coherent CoMP requires the network to know the state of the downlink channels allocated to each mobile station from a number of network antennas that are received strongly by each mobile station. In FDD (Frequency-Division Duplexing), the channel state information (CSI) is transmitted from each mobile station to the network on the assigned uplink channel. For an OFDM (Orthogonal Frequency-Division Multiplexing) system with FDD, let W denote the total number of subcarriers in the system, N the number of users reporting CSI per cell, B the average number of bits required to report the channel state information on each subcarrier, and T denote the reporting period used by each mobile station to report the channel state information.
Conventional techniques for enabling coherent CoMP require each mobile station to measure the downlink channel on all W subcarriers in the system and report the measurement, perhaps after compression, to the network every T seconds using the uplink channel. According to these conventional methods, the total average number of information bits B transmitted on the uplink for providing channel state information from all of the users to the network per cell is given by:RT=(W*N*B)/T  (1)
From equation (1), it is clear that as the number of users per cell increases, the total feedback overhead, RT, for conventional methods of reporting channel state information has a corresponding linear increase as a function of the number of new users. For a large number of users, the increased feedback load associated with reporting channel state information results in less uplink channel resources being available for transmission of user data on the uplink. Similarly, the spectral efficiency and cell-edge bitrate of an un-coordinated cellular system can be improved significantly if the network knows the state of the downlink channels to each mobile. The total feedback load for reporting channel state information fluctuates as a function of the number of users serviced by an uncoordinated base station which services an individual cell. That is, more uplink resources are consumed for reporting channel state information as the number of users serviced by the uncoordinated base station increases. Similar uplink resource fluctuations occur in TDD (Time-Division Duplex) based systems when the users are relied on to report some form channel state information using the uplink channel.